How a Coalescing Filter Works
A coalescing filter does not "strain" mist out — the droplets are far smaller than the gaps in the media. It captures them by three different physical mechanisms, each dominant in a different particle size range.
Drag the slider through the size range and watch how the droplets behave — and how many the media manages to catch.
Direct inertial impaction
above 2 µm
Large droplets carry too much momentum to follow the air as it curves around a fibre. They keep going straight and hit the fibre. Bigger and faster = easier to catch.
Interception
0.2 µm to 2 µm
Mid-size droplets do follow the airflow around the fibre, but the streamline passes within one droplet-radius of the surface, so the droplet touches the fibre and sticks. It is caught not by momentum but by geometry.
Diffusion (Brownian motion)
0.001 µm to 0.2 µm
The smallest droplets are so light that they are knocked around randomly by air molecules. They wander off the streamline and collide with a fibre by chance. Slower air = more time to wander = better capture.
This is why the hardest particle to capture is not the biggest — it is the one around 0.1–0.3 µm.
Below roughly 0.1 µm diffusion works well; above roughly 0.3 µm interception and impaction work well. In between, all three mechanisms are weak at once. That size is called the Most Penetrating Particle Size (MPPS), and it is the size filters are rated at, precisely because it is the worst case.
Once captured, the droplets merge (coalesce) into larger and larger drops until they are heavy enough to drain down and out of the element — that is where the name comes from.
Efficiency curve
Qualitative illustration of the characteristic efficiency dip around 0.1–0.3 µm — the shape is what matters, not the absolute values. Minimum shown at about 0.15 µm.
What this means when you buy a filter
A filter rated at 0.01 µm is being rated at its worst case — the rating describes the size it struggles with most, not the size it can only just manage. And a smaller rating is not free: it captures more, but it blocks faster and costs more pressure drop for as long as it is installed.